Two different organisms come to mind. The first is the leech. A
great deal of neurobiology has been done on this organism but I don't
know where it lies on your primitveness scale with relation to elegans
and drosophila. The other organism that may be worth considering is
xenopus. It has been used in at least one instance that I know of as
an intermediate to go from drosophila to human but I don't think a
great deal of neurobiology has been done on it.
Good luck.